Equalization of audio data is well known. Audio equalizers generally fall into one of three categories: (a) tone controls; (b) parametric equalizers; and (c) graphic equalizers. These categories are well-known and are therefore discussed only briefly below.
Tone controls use very small amounts of processing to raise or lower the audio frequency response that results from processing by a device. The increase or decrease of the frequency response is usually limited to two, or sometimes three, very broad bands of frequencies that are typically referred to as “Bass”, “Mid” and “Treble”. Tone controls are generally provided to allow an individual listener to control, with basic adjustments, the frequency response of the device to match their personal preference.
Parametric equalizers use a small amount of processing to generate effects such as simple bumps, dips, shelves, and high and low cut filters to modify sounds, often during audio recording and mixing. These individual audio effects (or modifications to the frequency response of the audio processing) are generally controllable by a number of continuously variable “parameters” (hence the name, “parametric equalizer”). For example, a bump effect is likely to be controllable by the following parameters: (i) gain, i.e. the amount of increase or decrease in frequency response; (ii) centre frequency, i.e. the frequency at which the magnitude of the increase or decrease in frequency response due to the bump is to be largest; and (iii) bandwidth (also known as ‘Q’, which is inversely proportional to bandwidth), i.e. the range of frequencies to be affected by the bump. Due to their low number of parameters, parametric equalizers can generally be quickly operated to achieve a desired effect on a particular audio source. However, parametric equalizers are generally not effective at achieving gentle audio effects across the whole audio band (or spectrum), nor are they effective at achieving a detailed but essentially arbitrary audio effect for room- or loudspeaker-correction.
Graphic (or graphical) equalizers use more processing than the above-mentioned tone controls and parametric equalizers. Graphic equalizers effectively divide the audio spectrum into a plurality (normally 31 or 61) of nominally independent, exponentially spaced, predetermined frequency bands. A gain control is then provided for each of these frequency bands. When the gains for these bands are controlled by vertical faders and these faders are arranged left to right in an increasing order of the frequencies they control, then the visual impression given is that the positions of the faders plot the frequency response produced by the graphic equalizer. Graphic equalizers are most frequently used to correct for problems in the final acoustic delivery of sound, such as problems in the loudspeaker, the room or both.
Graphic equalizers suffer from a number of problems that are mainly caused by the inevitable interaction between adjacent frequency bands. For example, if several adjacent faders are used to raise or lower the frequency response over a range of frequencies, then the resulting frequency response change is much greater than that actually indicated by the fader positions. This leads to excess effect over and above that intended by the user operating the faders. Furthermore, when several adjacent faders are used together in this way, the individual frequency bands do not combine smoothly to create a flat (either raised or lowered) frequency response. Instead, an unintended and subjectively annoying frequency response ripple is produced across these frequency bands. This makes it difficult to use a graphic equalizer to correct a room that presents problems over a broad range of frequencies.
Additionally (and at the opposite extreme to the above problems) it is difficult to correct or adjust the frequency response for a narrow band of frequencies using a graphic equalizer, since the individual filters which make up the graphic equalizer are at predetermined frequencies. These fixed frequencies cannot therefore be changed to the specific frequencies that the user wishes to correct.
Furthermore, due to the significantly increased amount of processing inherent to graphic equalizers when compared to tone controls and parametric equalizers (due to the combination of the outputs of a large number of filters), the phase response of a graphic equalizer is normally much more disruptive than is actually desirable.
It would therefore be desirable to provide improved audio equalization and filtering that addresses these problems.